Case Study: Omnivore Research Engine

Omnivore Research Engine

The OMNIVORE engine concept features an innovative variable compression ratio system and uses a two-stroke operating cycle with direct fuel injection. It is ideally suited to flex-fuel operation with a higher degree of optimisation than is possible with existing four-stroke engines.

The engine concept features a monoblock construction that blends the cylinder head and block together eliminating the need for a cylinder head gasket, improving durability and reducing weight. In this case, the application of a monoblock is facilitated by the absence of the requirement for poppet valves. A novel charge trapping valve in the exhaust port allows asymmetric timing of exhaust flow and continuous variation of the exhaust opening timing.

The variable compression ratio is achieved by the use of a puck at the top of the combustion chamber. This simple, yet effective system moves up and down effecting the change in geometric compression depending on the load demands on the engine.

The monoblock incorporates the cylinder head, the cylinder barrel and the inlet ports, together with mounts for the variable compression ratio system and the charge trapping valve housing. It also contains the non-moving location of one of the two possible injector mounting positions provided for research purposes. The other injector position is in the variable compression ratio puck. The monoblock is mounted on the upper crankcase, which is a common component with all of Lotus single-cylinder research engines. The engine carries a full primary and secondary balancer system. The monoblock is water-cooled by an electric water pump.

Computational fluid dynamics is used extensively to ensure effective cooling of the monoblock, a feature assisted by the removal of the cylinder head gasket, inherent in such architecture. The chief advantage of a monoblock construction in any engine, aside from the bill of materials and assembly benefits, is the reduction of bore distortion afforded by the removal of cylinder head bolts. This is especially important in piston-ported 2-stroke engines.

OMNIVORE ANIMATION

VARIABLE COMPRESSION RATIO MECHANISM

The primary component of the variable compression ratio mechanism is what is termed the 'puck' or a moveable junk piston in the cylinder head. In the case of the research engine, this puck is driven in and out by a double-eccentric mechanism itself comprising proprietary parts. The puck itself does not move at engine speed. In addition to the spark plug, the puck carries one of two possible injector positions. It is water-cooled and carries simple piston (or 'junk') rings for primary sealing, and an 'O'-ring towards the top for final sealing.

The variable compression ratio system is controlled by an electric motor and worm drive arrangement at the front of the engine. Because there are no poppet valves in the engine, it is clear that the puck could be of a large diameter and since there is no need for valve cut-outs in the piston crown, the minimum volume of the combustion chamber can be much smaller than has been the case in variable compression ratio engines shown to date. When the puck is in its innermost position, its surface is essentially coincident with that of the combustion chamber squish band and this yields the highest compression ratio of 40:1.

The combustion chamber geometry necessarily alters as the puck is moved to vary the compression ratio. The chamber geometry in OMNIVORE was therefore chosen on the basis of 2-stroke experience in spark ignition operation. Consequently, the puck is positioned in the cylinder head in such a way that the non-moving squish band directs cooling flow towards the spark plug. The puck is water-cooled from the main engine cooling circuit.

CHARGE TRAPPING VALVE

The charge trapping valve is caused to oscillate by a short articulated connecting link from an engine-speed eccentric shaft itself rotated by a belt drive from the crankshaft. A simple charge trapping valve mechanism provides for asymmetric exhaust timing and hence a modification of the original piston-ported two-stroke operating cycle. Fitting an articulated link between the eccentric shaft and the trapping valve actuating arm affords the opportunity independently to vary the opening and/or closing point. In this 'variable' form, at light load, the charge trapping valve can be made to control exhaust port opening, to maximise expansion in the cylinder, and the blowdown period can be optimised. The position of the control arm is controlled by the engine management system. All charge trapping valve components and their configuration have been analysed kinematically, and since they operate with modified simple harmonic motion, they do not suffer from jerk stresses.